This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s) 2002-239680 and 2002-239685 filed in JAPAN on Aug. 20, 2002, which is(are) herein incorporated by reference.
(1) Field of the Invention
The present invention relates to a sheet feeder for use in an image forming apparatus such as a printer, facsimile machine, copier etc., to feed sheets to the image forming apparatus, and also relates to an image forming apparatus having the same.
(2) Description of the Related Art
In a typical image forming apparatus such as a copier, printer etc., recording sheets (paper) are delivered from paper feed trays arranged in the lower part of the machine and fed to the image forming portion by way of the conveyance path. Here, the paper feed trays are receptacles which hold paper to be used for image forming. These paper feed trays hold sheets of regular sizes which are easy to form images thereon, perform beneficial conveyance and are used often.
In image forming apparatuses, however, in some cases, thin sheets, thick sheets such as post cards, sheets of various colors (colored sheets), sheets of uncommon sizes, sheets of special materials and so on may be used as recording sheets, in addition to the paper held in the paper feed trays. Such recording sheets which are rarely used (to be referred to hereinbelow as uncommon sheets) may be difficult to handle using the aforementioned paper feed trays, and it is not efficient that such uncommon sheets monopolize the limited number of paper feed trays at all times. However, it is also troublesome if recording sheets held in the paper feed tray should be replaced every time one type of uncommon sheets are used. For this reason, many of image forming apparatuses have a paper feed port on the outside (flank) of the machine, i.e., so-called manual paper feeder (an example of the sheet feeder) so as to facilitate uncommon sheets to be fed to the image forming apparatus. This manual paper feeder has a pickup device (an example of the feeding device) which places and presses a designated pickup roller onto the topmost surface of recording sheets stacked on the manual feed tray as a recording sheet receiver and turns it so as to pick up recording sheets, one by one, and feeds them into the image forming apparatus.
With recent utility enlargement of image forming apparatuses, more types of recording sheets have become used in the image forming apparatuses. Further, with the trend toward high-speed image forming, the number of sheets being able to be set at a time reaches some hundreds, and the capacity of the manual feed tray tends to be greater.
When the capacity of the manual paper feeder is made large, the conditions for picking up recording sheets vary significantly between the state where the feeder is set full of sheets and the state where the feeder is set with the minimum amount of paper or with only one sheet. Specifically, when the weight of the pickup device that is pivoted on the flank of the image forming apparatus or some repulsive force such as a spring or other elastic element is used to press the pickup roller against the recording sheets, the pickup strength varies depending on the position (height) of the pickup roller. Illustratively, the position or height of the pickup roller changes greatly between the state where many recording sheets are set and the state where a few of recording sheets are set, and this results in a large change in pickup strength. In general, when a lower amount of recording sheets are stacked (when the pickup roller is set low), the pickup strength lowers resulting in failure to pick up recording sheets.
Conventionally, in order to avoid this problem without limiting the number of recording sheets being set to a lower number, the drive force of a predetermined drive source is transmitted to the pickup device so as to urge the pickup roller against the recording sheets with a fixed force. Thereby the pickup strength of the pickup roller is made stable regardless of the number of recording sheets, thus making it possible to prevent pickup failures.
Since the manual feed tray and the pickup device which constitute this manual paper feeder are arranged so that they project from the flank of the apparatus body, the full width of the whole apparatus becomes large, causing a problem of excessive bulk. To deal with this problem, Japanese Patent Application Laid-open Hei 11 No. 171360 discloses an image forming apparatus configuration in which when the manual paper feeder is not used or when the image forming apparatus needs to be moved the manual feed tray is set upright along the flank of the image forming apparatus so as not to cause hindrance and with this movement, the pickup device is retracted into the image forming apparatus body so as not to interfere with the manual feed tray.
Here, since the pickup roller in the manual paper feeder is arranged projecting from the flank of the image forming apparatus, it is liable to be stained with sweat and grease or be damaged when accidentally touched by an operator's hands or by some other event. When the pickup roller is stained or damaged, the pickup performance may lower. In order to solve this problem, when the pickup device is arranged outside the machine body, it is necessary to prevent the pickup roller from being stained or damaged by providing a protecting cover for shielding the device.
However, this configuration with the pickup device and protecting cover provided outside the apparatus body entails the problem that when the pickup device is attempted to be retracted into the flank of the image forming apparatus body, the pickup device cannot be snugly retracted because the protecting cover stands in the way.
Further, in the case where the pickup strength is stabilized with the help of the drive force from a drive source as stated above, since the pickup device needs to be rotated opposing the static force of the drive source when the pickup device is retracted into the flank of the image forming apparatus, this configuration poses the problem that a very strong force is needed.